1. Field of the Invention
The invention relates to a battery remaining capacity measuring apparatus and more particularly to a battery remaining capacity measuring apparatus according to a battery temperature.
2. Description of the Related Art
In general, since an electric car moves using a battery, it is necessary to estimate a battery remaining capacity. The followings are systems for measuring a battery remaining capacity.
(1) A system for measuring and integrating a quantity of a current provided from a load side, and estimating a remaining capacity according to the current integrated value and a quantity of a current while charging, namely, a current integrating system.
(2) A system for measuring a terminal voltage when a discharge current is applied to a load side from a battery terminal and estimating a remaining capacity, namely, a voltage detecting system.
(3) A system for measuring a specific gravity of an electrolyte in a battery and estimating a remaining capacity, namely, a specific gravity of an electrolyte system.
(4) A system for disconnecting a battery from a load, estimating a terminal voltage in the state of an open circuit and estimating a remaining capacity, namely, an open circuit voltage detecting system.
(5) A system for measuring a change of an internal resistance in a battery and estimating a remaining capacity, namely, an internal resistance measuring system.
As described above, there are some systems for measuring a remaining capacity and so on.
For example, according to a voltage detecting system as described in (2), in general, a full-charged voltage V.sub.F corresponding to a capacity (100%) on full-charging and a voltage V.sub.E (as refereed to a voltage when a remaining capacity is 0% below) when a capacity of a battery is 0% are previously set. An initial voltage (for example, a voltage when full-charging) while disconnecting a load is detected. According to the initial voltage, a remaining capacity voltage is estimated after this.
For example, if a detected voltage (refereed to as an estimate voltage V.sub.N below) becomes a full charged voltage V.sub.F, a display is that 100% of a remaining capacity remains. While, if an estimate voltage V.sub.N becomes a voltage V.sub.E when a remaining capacity is 0%, a display is that 0% of a remaining capacity remains.
However, a battery voltage is inclined to decrease as a temperature drops.
Thus, as shown in FIG. 1, according to a relationship between a battery capacity and an estimate voltage V.sub.N, an estimate voltage V.sub.N is below a predetermined full-charged voltage V.sub.F, even if 100% of a battery capacity remains actually. While, even if 100% of a battery capacity remains, a display is that less than 100% of a battery capacity remains.
Furthermore, a remaining capacity voltage is estimated according to an initial voltage and a detected voltage. Accordingly, when a temperature drops, even if a battery capacity does not remain, an estimate voltage V.sub.N is more than a voltage V.sub.E of when 0% of remaining capacity remains. A display is that more than 0% of a battery capacity remains.
While, when a temperature is high, a battery voltage is inclined to get higher.
Thus, as shown in FIG. 2, even if less than 100% of battery capacity remains actually, an estimate voltage V.sub.N is more than a full-charged voltage V.sub.F, and a display is that a remaining capacity is 100%.
Furthermore, a remaining capacity voltage is estimated according to an initial voltage and a detected voltage. Accordingly, when a temperature goes up, even if a battery capacity still remains, an estimate voltage V.sub.N is a voltage V.sub.E of when a remaining capacity is 0%. A display is that 0% of a battery capacity remains.
That is, since a battery voltage changes according to a temperature, it is necessary to correct a battery temperature so that a correct remaining capacity can be obtained.
By the way, a battery temperature changes due to various causes, and a discharge characteristic differs as a temperature changes.
In general, it is necessary to obtain a predetermined number of temperature data, voltage data, current data and so on, to use a method of least squares and so on, to find a preferable temperature correcting coefficient, and to correct an estimate capacity voltage. Accordingly, a program configuration of an apparatus is complicated.
That is, the problem is that it is not easy to correct an estimate capacity voltage according to a battery temperature and to find a correct remaining capacity.